do I need to get a visa to mainland PRC??

Kath, if after visiting mainland PRC for a day, you will fly back to HK, I’m afraid you cannot enjoy the free transit. You should apply for an entry permit in advance.

Can I get a 72 hours visa free transit in HK?

You mean the 72 hour free transit in mainland PRC, right? If you stay in mainland PRC for one day and then go back to HK, you cannot use the free transit and please note that the 72 hour free transit is mainly applicable to those travelling by flight.
But after one-day stay in mainland, if you fly to a third country or region, instead of HK, you can enjoy the direct transit. In this case, no entry permit is required.

Thank you. You have been most informative. I will apply for a visa before I leave uk.